A power cylinder assembly of an internal combustion engine generally includes a reciprocating piston disposed within a cylindrical cavity of an engine block. One end of the cylindrical cavity may be closed while another end of the cylindrical cavity is open. The closed end of the cylindrical cavity and an upper portion or crown of the piston defines a combustion chamber. The open end of the cylindrical cavity permits oscillatory movement of a connecting rod, which joins a lower portion of the piston to a crankshaft, which is partially submersed in an oil sump. The crankshaft converts linear motion of the piston (resulting from combustion of fuel in the combustion chamber) into rotational motion.
Internal combustion engines, and in particular the pistons of such engines, are under increased stress as a result of efforts to increase overall efficiency, e.g., by reducing piston weight and/or increasing pressures and temperatures associated with engine operation. Thus, to improve engine performance, increase engine efficiency, and reduce fuel consumption, engine designs have been reduced in size in recent years. As engine size has reduced, combustion temperatures have correspondingly and generally increased. Piston cooling is therefore increasingly important for withstanding the increased stress of such operational conditions over the life of the engine.
Known piston designs include a combustion bowl facing the combustion chamber, the combustion bowl typically having a curved shape that optimizes power output of the piston during the combustion process. That is, typically the shape of the combustion bowl is selected such that the flame front grows optimally into the curved combustion bowl during each combustion event to maximize power output.
Known piston designs also typically include cooling galleries disposed approximately about a circumference of the combustion bowl, allowing for coolant fluid to pass through and remove heat during piston operation. Crankcase oil is introduced into the cooling gallery, and the coolant removes combustion energy that passes via conduction into the piston. However, in comparison to the very rapid combustion process, conduction to the cooling gallery is relatively slow. Thus, although the crankcase oil passing through the cooling gallery may be sufficient to remove the heat passing into the piston, peak temperatures occur on a surface of the combustion bowl during the combustion process well in excess of the temperature of the crankcase oil.
It has been used in the past to provide thermal barrier coating with various ceramics that have low thermal conductivity properties, but the end result had poor life due to cracking and failure of the ceramic coating and in particular the coating adhesion to the piston body. In addition, the surface temperature may increase due to the coating resulting in inefficiencies and possible knocking concerns.
Accordingly, there is a need for a robust, lightweight piston design that allows adequate cooling, such as by providing a cooling gallery, while simplifying construction of the piston, while increasing the adhesion and durability of the thermal coating.